1. Field of the Invention
The present invention relates to a ceramic filter comprised of a porous support made of a ceramic and, formed on one surface thereof, a thin film having pores finer than the pores of the support. It also relates to a process for manufacturing the same.
2. Description of The Related Art
As a ceramic filter used in the ultrafiltration, reverse osmosis or gas separation of pharmaceuticals or food, a filter is known which is comprised of a porous support made of a ceramic and, formed on one surface thereof corresponding to the side from which a substrate solution is flowed in, a thin film having pores (e.g., 200 .ANG.) finer than the pores of the support, as disclosed in, for example, Japanese Laid-open Patent Application No. 59-62324, No. 59-102403, No. 59-109203 or No. 60-156510.
The above thin film comprises particles with a diameter smaller than the particles constituting the support, and is formed by preparing an alumina sol, a Boehmite sol or a colloidal silica or porous silica in the form of a sol which are in the form of hydrates formed by alkoxide hydrolysis, aqueous solution precipitation or the like, and applying any of these sols to the surface of the support, which is then dried to make it into a gel, followed by baking.
In the above conventional ceramic filter, the gel particles, when baked in the course of the formation of the thin film, may grow to give a large diameter, or this may make the thin film amorphous, and hence the resulting thin film can not be said to have a sufficient physical and chemical strength. To cope with this, the present applicant has disclosed in Japanese Laid-open Patent Application No. 2-126924 a ceramic filter in which particles of SnO.sub.2, TiO.sub.2 or ZrO.sub.2 are used as gel particles for the formation of the thin film and are baked at a low temperature.
Selection of gel particles and also baking them at a low temperature as in the above enables formation of a thin film having a small pore size and a superior durability. However, in the case when the thin film is used as an ultrafiltration membrane or the like, the permeability is also important from the viewpoint of operating efficiency. An improvement is sought in this respect.
More specifically, the permeability can be improved if the thin film is made to have a thickness of as small as 1 .mu.m or less and the particles constituting the thin film are made to have a smaller diameter. An excessively small film thickness, however, may cause the surface of the support, the ground, to be partially uncovered, or may result in an excessively great difference in particle diameter between the particles constituting the thin film and the particles constituting the support, tending to bring about peeling or cracks.